Generally, photovoltaic devices employed in electrical power generation are arranged as arrays of individual cells interconnected by conductive bands or layers (e.g. metal or Transparent Conductive Oxides etc) to produce a series parallel connected array.
Series connection of individual cells is employed to provide higher output voltages than can be achieved from a single cell, while parallel connection of cells boosts output current. The provision of both series and parallel connection of cells within a device, with relatively high levels of inter-connection between parallel paths, has the added effect of minimising degradation of the device performance due to temporary or permanent degradation of performance in an individual cell or group of cells. For example, dirt or bird droppings may reduce the current produced by cells in a small region of a device and will restrict the current flowing in all cells connected in series with those cells. However, parallel connections to other series connected cells will enable current to flow around the affected cells and minimise the effect on the total device.
It is also common practice, due to the relatively lower conductivity of semiconductor material in the body of a cell when compared to the contact material, to provide multiple contacts over a surface of individual cells to reduce series resistance in the device by reducing the distance that carriers in the semiconductor material are forced to flow laterally before reaching a contact.
Over any large area photovoltaic module, there will be shunts or short-circuits scattered about the module. Current drawn by these shunts is directly subtracted from current that could be delivered to the load. In thin film modules, the shunts typically occur at tiny bridges between metal contacts (for example due to a contaminant locally preventing the formation of a break in a metal contact layer), or due to pinholes in the insulating layers that are intended to separate the metal contact layers from the underlying semiconductor. The problem caused by shunts and localised reductions in device performance is reduced with increased levels of serial and parallel connection of cells within the module, however this also implies an increase in complexity of the metallisation patterning.
The cost of manufacturing devices with complex contact patterns is a significant factor In the overall cost of device manufacture, and is increased with Increased pattern complexity, due to the need for accurate alignment steps and in the case of laser patterning a significant reduction in speed over that possible with less complex patterns.